1. Field of the Invention
The present invention relates to a bearing with dynamic pressure grooves used for office equipment, acoustic equipment, measurement equipment, and the like, and to a method for manufacturing the same.
2. Description of the Prior Art
A prior art cylindrical slide bearing made of a synthetic resin, in particular, a bearing having dynamic pressure grooves for generating a dynamic pressure formed in an inner peripheral surface, is manufactured by injection molding in a mold by using a compound material containing an elastic thermoplastic mixed with a lubricating material, such as graphite, carbon fiber, molybdenum disulfide, fluoroplastic, or the like, and by drawing the bearing out of the mold by utilizing the elasticity of the resin. However, as to a bearing having dynamic pressure grooves using a thermoplastic other than a PTFE (polytetrafluoroethylene), although it is possible to manufacture such a bearing, molding shrinkage is large, dimensional tolerances are not sufficient, and friction properties are not satisfactory. Thus, the performance required for the bearing with dynamic pressure grooves is not sufficiently met.
In contrast, Japanese Patent Laid-Open Application No. 63-203916 (hereinafter referred to as a first prior art example) proposes a bearing with dynamic pressure grooves using a thermosetting plastic and exhibiting a high accuracy, and a method for manufacturing the same. This bearing with dynamic pressure grooves includes a thin wall inner cylindrical body made of a thermosetting plastic, such as an epoxy resin, phenolic resin, unsaturated polyester resin, diallylphthalate resin, or the like, and formed with grooves for generating a dynamic pressure in an inner peripheral surface. This bearing further includes a metallic outer cylindrical body. The thin wall inner cylindrical body is secured to the inner peripheral surface of the outer cylindrical body.
In manufacturing this bearing with dynamic pressure grooves, an outer cylindrical body formed with a plurality of rows of recessed grooves in an inner peripheral surface, or applied with a securing means, such as an adhesive or the like beforehand, is fitted into an outer mold. Following this, a molding material of a heated and melted thermosetting resin is supplied into an annular space having a narrow width between the outer cylindrical body fitted into the outer mold and an inner mold having ridges corresponding to the shape of the dynamic pressure generating groove formed and arranged on an outer peripheral surface thereof. An inner cylindrical body is formed by hardening the molding material, and at the same time, the inner cylindrical body is made to be secured to and held by the outer cylindrical body by the securing means thereby to form a laminated structure having the inner cylindrical body and the outer cylindrical body integrally coupled together. Thereafter, the laminated structure of the inner cylindrical body and the outer cylindrical body is drawn out of the outer mold and the inner mold in an axial direction so that the laminated structure is separated from the outer and inner molds.
Japanese Utility Model Laid-Open Application No. 60-93012 discloses a bearing with dynamic pressure grooves (hereinafter referred to as a second prior art example) in which dynamic pressure grooves are formed in one surface of a flat plate by etching or plastic work in advance. This flat plate is then wound in a shape which matches the shape of an outer peripheral surface of a shaft, or the shape of an inner peripheral surface of a supporting member of the shaft, and the opposite ends or edges of the flat plate are butted and joined to each other to thereby form a sleeve. The sleeve is fitted about and secured to the outer peripheral surface of the shaft, or fitted into and secured to the inner peripheral surface of the supporting member, to form a fluid bearing.
Furthermore, Japanese Utility Model Laid-Open Application No. 63-27143 discloses a bearing with dynamic pressure grooves (hereinafter referred to as a third prior art example) in which a shaft body and a bearing are made to move relative to each other in a forward and a reverse direction through mutual slide surfaces. At least one of the slide surfaces is formed with dynamic pressure generating grooves having an arrowhead shape. This bearing is a dynamic pressure type slide bearing. The dynamic pressure generating grooves include one group of grooves whose arrow tip direction is coincident with one of the directions of movement of the shaft body and the bearing and another group of grooves whose arrow tip direction is coincident with the other of the directions of movement of the shaft body and the bearing. As a result, this bearing is a slide bearing in which the dynamic pressure is generated in both forward and reverse directions, that is, in both one and the other directions of the movement of the bearing.
However, in the first prior art example mentioned above, although the thermosetting resin has an excellent molding accuracy, a problem is involved in that the resin itself has poor friction properties and wear-resistant properties and, even when a lubricating material, such as graphite, carbon fiber, molybdenumdisulfide, or the like, is mixed with the thermosetting resin, satisfactory friction properties and wear-resistant properties cannot be obtained.
Furthermore, another problem is involved when a plurality of unitary bearings are assembled to form a unit assembly. In this structure, it is difficult to arrange the plurality of bearings so that inner diameters thereof are exactly coaxial to one another or to obtain the desired coaxiality.
Also, the second prior art example is the fluid bearing of a radial type in which the shaft is supported by a dynamic pressure of fluid generated in the dynamic pressure generating grooves by rotation of the shaft and the pressure generating grooves are formed continuously over the entire circumference of the outer peripheral surface of the shaft, or the inner peripheral surface of the support member. However, this bearing is manufactured in such a manner that, first, the sleeve is formed by winding a flat plate having dynamic pressure generating grooves formed beforehand in one surface, and by butting and joining opposite ends of the plate. The sleeve is then fitted about and secured to an outer peripheral surface of a shaft, or is fitted into and secured to an inner peripheral surface of a supporting member. As a result, a problem is involved in that the manufacturing process is complicated and the manufacturing cost is high. Furthermore, a uniform quality is difficult to achieve because of variations of the length of the cut flat plate, roughness of the butting portions of the opposite ends of the flat plate, nonuniformity in fitting of the formed sleeve about the shaft or into the supporting member, and the like.
On the other hand, in the dynamic pressure type slide bearing in the third prior art example, in the case wherein the mutual slide surfaces of the shaft body and the bearing are cylindrical, the dynamic pressure generating grooves of an arrowhead shape are formed over the entire circumference of at least one of the slide surfaces. However, it is difficult to manufacture this dynamic pressure type slide bearing on the basis of mass production.